Spreading of a signal over multiple time/frequency resources is performed for the purpose of achieving higher diversity, reliability and robustness to interference and channel variations.
Code division multiple access (CDMA) is a multiple access technique in which data symbols are spread over orthogonal or near-orthogonal code sequences. Conventional CDMA encoding is a two-step process in which a binary code is mapped to a quadrature amplitude modulation (QAM) symbol before a spreading sequence is applied. CDMA can be considered a form of repetition of QAM symbols by using different amplitudes and phases in accordance with the spreading sequence.
Conventional CDMA encoding can provide relatively high throughput. However, new techniques/mechanisms for achieving even higher throughputs may be desirable to meet the ever-growing demands of next-generation wireless networks. Low density spreading (LDS) is a form of CDMA used for multiplexing different layers of data. LDS uses repetitions of the same symbol on layer-specific nonzero positions in time or frequency. As an example, in LDS-orthogonal frequency division multiplexing (OFDM) a constellation point is repeated over nonzero frequency tones of an LDS block. This sparse spreading helps reduce the decoding complexity when many (e.g., more than the spreading length) LDS signals are multiplexed together.
In sparse code multiple access (SCMA), a multidimensional codebook is used to spread data over tones without necessarily repeating symbols. In SCMA the multidimensional spreading codebooks are sparse and hence detection can be made simpler. SCMA allows the encoding of data using a subset of tones from a larger set, so as to improve overall performance when viewed as a trade-off between error rate and throughput. One of the properties of SCMA is that larger and more complex constellations can be formed from multiple tones. However, although this may increase throughput, it also consumes valuable computational resources and may perform sub-optimally with low complexity receivers.
Therefore, there is still a need for non-orthogonal multiple access solution that maintains the good performance that is due to efficient multi-dimensional modulation, and is flexible for addressing various next generation application scenarios and multi-user multiplexing.